Heterodyne detection refers to detection in which a received signal is mixed with a local oscillator signal to generate an intermediate frequency (IF) for further processing. An example of heterodyne optical spectral analysis is discussed by Baney et el. in IEEE Photonics Technology Letters 14(3) (March 2002). FIG. 1 depicts coherent optical spectrum analyzer 100 according to the discussed design. A optical signal (denoted by ES(t)) to be analyzed may be provided to an input fiber of 2×2 optical coupler 101. Local oscillator 102 may provide a local oscillator signal (denoted by ELO(t)) to the other input fiber of optical coupler 101. Optical coupler 101 superimposes the optical signals (ES(t) and ELO(t)). The output signals from optical coupler 101 (denoted by EA(t) and EB(t), respectively) are utilized to illuminate respective photodetectors 103a and 103b (e.g., photodiodes). Amplification is performed by transresistance amplifers 104a and 104b. The output from one of transresistance amplifiers 104 is subtracted from the other transresistance amplifier 104 by combiner 105.
The heterodyne signal is given by A(t)cos(2πΔft+Δφ(t)) where A is the heterodyne amplitude, Δf is the heterodyne beat frequency, and Δφ is the heterodyne phase. The heterodyne amplitude (A) is related to the power of the local oscillator and the power of the optical signal. The heterodyne beat frequency (Δf) is given by the instantaneous difference in frequency between the frequency of the optical signal and the frequency of the local oscillator. Likewise, the heterodyne phase (Δφ) is given by the instantaneous difference in phase between the phase of the optical signal and the phase of the local oscillator.